1. Field of the Invention
The present invention relates to an engine starting system for automatically stopping an engine when predefined conditions for automatic engine stop are satisfied in an engine idling state, for instance, and restarting the engine when conditions for engine restart are satisfied after automatic engine stop.
2. Description of the Related Art
Automatic engine stop control technology, or so-called idle stop control technology, has been developed in recent years, aiming at reducing fuel consumption of and carbon dioxide (CO2) emissions, for instance, from an engine. This technology automatically stops the engine at idle, for instance, and automatically restarts the engine at a point in time when engine restart conditions are satisfied as a result of a driver's action taken after idle stop for starting up a vehicle, for instance. Generally, engine restart performed during idle stop control operation requires promptness to instantly restart the vehicle in response to a vehicle start-up action, for instance. If a widely practiced conventional method of engine restart in which the engine is restarted by cranking an output shaft of the engine by means of a starter motor is used, however, there arises a problem that a good deal of time is needed up to completion of an engine start sequence.
Thus, it is desirable to supply fuel into a cylinder which has stopped on an expansion stroke at Idle stop and cause ignition and combustion to occur in that cylinder so that the engine is instantly started with resultant combustion energy. However, if the position where a piston in the cylinder which has stopped on the expansion stroke is inappropriate, that is, if the piston has stopped at top dead center or at a position extremely close to bottom dead center, for instance, the quantity of air in the cylinder is significantly small so that it is impossible to obtain a sufficient amount of combustion energy, or the distance traveled by the piston while the combustion energy is exerted on the piston Is so small that it may be impossible to normally restart the engine.
There exist conventional approaches aimed at solving the aforementioned problems. For example, Japanese Laid-open Utility Model Publication No. 1985-128975 proposes an engine starting system in which a crankshaft of an engine is provided with a braking device which is controlled in such a way that the piston in a cylinder which stops on the expansion stroke would halt at an appropriate position halfway on the expansion stroke.
Also, Japanese Laid-open Patent Application No. 2001-173473 discloses an engine starting system which, when it is judged that automatic engine stop conditions have been satisfied, increases intake air pressure to boost compressive pressure produced in a cylinder of which piston halts on the expansion stroke so that the piston in the cylinder stops at a specified position.
Further, Laid-open European Patent Application No. 1403512 shows an engine starting system which, when it is judged that automatic engine stop conditions have been satisfied, increases the quantity of intake air for a specified period of time to regulate the quantity of intake air so that the piston in the cylinder stops at a specified position while accelerating exhaust gas scavenging operation.
According to the engine starting system proposed in Japanese Laid-open Utility Model Publication No. 1985-128975, it is necessary to provide a device for braking the crankshaft of the engine in addition to an ordinary vehicle braking device. Additionally, it is necessary to control the crankshaft braking device with high precision in order that the piston in the cylinder which stops on the expansion stroke would settle at the appropriate position. A problem of this engine starting system is that it is so difficult to control the crankshaft braking device with the desired precision.
On the other hand, the engine starting system of Japanese Laid-open Patent Application No. 2001-173473, which is configured such that the in-cylinder compressive pressure is increased by boosting the intake air pressure at a point in time when the automatic engine stop conditions have been satisfied, has a problem that it is difficult to properly stop the piston at a position suited for engine restart if the degree of engine speed slowdown varies causing a change in the position where the piston stops. This engine starting system also has a problem that it is difficult to sufficiently improve exhaust gas scavenging performance of the engine at automatic engine stop.
By comparison, the engine starting system of Laid-open European Patent Application No. 1403512 causes the piston to stop at a position where a sufficient quantity of air has been introduced into the cylinder which stopped on the expansion stroke, so that piston movement up to a point of complete stop becomes more stable. Therefore, even if the degree of engine speed slowdown varies to a certain extent, this engine starting system can cause the piston at the specified position more easily. Moreover, since the exhaust gas scavenging operation is accelerated by temporarily increasing the intake air quantity, it is possible to ensure a sufficient quantity of fresh intake air even when restarting the engine immediately after engine stop.
However, the engine starting system of Laid-open European Patent Application No. 1403512 still has a problem that it is difficult to avoid changes in the piston stop position due to the influence of differences in intake system and other engine characteristics from one engine to another as well as variations in engine temperature and other parameters. Under such circumstances, it is desired to develop technology which makes it possible to suppress variations in the piston stop position and cause the piston to stop at an appropriate position in a more reliable fashion than ever.